The present invention relates to an apparatus and related method for simultaneously slitting multiple layers of corrugated paperboard or the like and, more particularly, to techniques utilizing a thin rotary slitting blade for longitudinally slitting an advancing shingle of multiple overlapping sheets of corrugated paperboard.
Apparatus for longitudinally slitting a continuous traveling sheet or web of corrugated paperboard is well known in the art. Typically, multiple slitting tools are mounted coaxially and are laterally spaced across the width of the sheet in selectively adjustable positions. Similarly, multiple scoring tools may also be coaxially mounted and spaced across the sheet width such that the traveling sheet may be scored and slit simultaneously.
In conventional prior art apparatus, each pair of upper and lower slitting tools is disposed with overlapping radial cutting edges between which the advancing paperboard sheet is moved to provide a continuous cut. Although the operation is commonly referred to as "slitting", the cutting tools in fact shear the moving sheet causing a relative vertical displacement of the cut edges from one another. As the thickness of the corrugated board increases, the cuts tend to become more ragged, the edges tend to be crushed, and the quality of the cut degrades significantly. Another problem commonly associated with rotary shear cutting of corrugated board is the generation of dust. Fine paper dust is a very serious problem which may result in clogged filters on air handling systems, hazardous air quality for workers, contamination of electronic and other equipment, and even explosion or fire hazards. Conventional shear-type slitting often simply cannot provide adequate slit edge quality for heavy board thicknesses or for multiple layers of board. As a result, attempts to utilize conventional shear-type slitting for heavy double and triple wall corrugated board fail because the edge quality is poor in appearance or is structurally unacceptable.
U.S. Pat. No. 4,627,214 shows a slitter-scorer apparatus of one prior art construction in which the board is sheared by passage between the overlapping edges of a pair of rotary cutting knives. Even when such cutting knives are properly adjusted and maintained in a sharpened condition, their use to slit heavier double and triple wall board has been less than satisfactory. Attempts to utilize this apparatus to cut multiple layers simultaneously would, similarly, result in unsatisfactory slit quality.
As a result of the limitations inherent in conventional shear-type slitting, the use of slitters has been relegated primarily to the slitting of a continuous sheet as it exists the corrugator. However, slitting could be employed at other stages in the manufacture of a corrugated paperboard box, if the problems with slit quality degradation with increasing board thickness and multiple layers could be remedied. For example, in certain applications it is desirable to longitudinally slit a partially assembled box or carton as it exits in a double layer configuration from a flexo-folder-gluer. However, attempts to utilize conventional slitting technology on such a double layer configuration has been less than satisfactory. Furthermore, conventional processing strategy utilizes shingling of serially processed sheets (such as partially assembled containers from the flexo-folder gluer) in order to reduce conveyor lengths and to facilitate stacking. If downstream slitting of shingled sheets is desired, however, the sheets must first be unshingled, slit singly, and then reshingled for further processing. This not only requires additional processing equipment, but also reduces processing speed. Furthermore, shear-type slitting of an assembled double layer container results in generally poor quality edges. For example, in the manufacture of two half slotted containers (HSC's) from a single regular slotted container (RSC) by slitting the latter longitudinally, it is customary to provide extra edge material to compensate for the collapse of the edge under stacking loads, the collapse being the result of slit edge degradation caused by shear-type slitting.
Another problem which has been encountered in the prior art and remains unsolved is maintaining registration of knocked down boxes, such as RSCs, so that the longitudinal dividing slit is always accurately located to maintain the transverse dimensions of the two resulting containers as precise as possible. Similarly, a persistent problem in the prior art is to retain the squareness of knocked down boxes which are printed, folded and glued in a flexo-folder-gluer, but which often become out-of-square as a result of downstream handling before the glue is set.
U.S. Pat. No. 3,599,518 discloses a device for longitudinally slitting a continuous sheet of corrugated paperboard which is intended to overcome the deficiencies inherent in conventional shear-type slitting, discussed above. In the device disclosed in this patent, a rotatable annular slitting blade slits the advancing sheet which is supported from below on a bristled brush roller into which the cutting edge of the blade penetrates as it passes through the paperboard sheet. The blade is rotated at a high overspeed with respect to the speed of the advancing paperboard sheet and the supporting brush roller is rotated at a slight overspeed with respect to the advancing sheet. The peripheral cutting edge of the blade is serrated and hard faced to retain sharpness. This patent also discloses the use of nozzles to discharge a mist of steam or oil downwardly onto the cutting blade from above to prevent the accumulation and drying of adhesive and paper dust on the blade or blades.
However, the use of a supporting brush roll has been found to have the following drawbacks. If the bristles are made stiff enough to provide adequate support for the paperboard sheet, the blade tends to cut the bristles. If the bristles are made more flexible to avoid cutting, adequate support for the sheet is lost. In addition, a serrated blade creates substantially more board dust and, in addition, serrated blades are difficult to sharpen and in any event must be removed from the machine for sharpening. Furthermore, spraying of any liquid onto the blade, particularly oil, inevitably results in unacceptable overspray which contaminates the board, the equipment and the atmosphere.
In copending and commonly assigned patent application Ser. No. 490,793, filed Mar. 8, 1990, now abandoned, there is described a slitting apparatus which overcomes many of the problems associated with prior art shear-type slitting. That apparatus, however, is particularly adapted to provide one or more longitudinal slits in a single continuous sheet, such as that exiting a corrugator. The apparatus disclosed in that application does not provide the capability for slitting multiple sheets, particularly a shingle of multiple overlapping sheets of corrugated paperboard.
Therefore, it would be most desirable to provide a method and apparatus for the simultaneous slitting of multiple overlapping layers of corrugated paperboard and, particularly, for the continuous slitting of paper in a shingled configuration. It would be especially desirable to have a method and apparatus which could accurately and cleanly slit shingled knocked down boxes out of a flexo-folder-gluer.